Oxidation of lignin in microemulsion reactor
Basing on the constructed microemulsion system and the above
characterizations, the lignin oxidation performance in the microemulsion
system using water soluble CuSO4 as the catalyst was
investigated (Table 2). In this study, the products from lignin
oxidation are classified to p-hydroxyphenyl (H), guaiacyl (G) and
syringyl (S) alcohol unit products according to their structure as it is
widely accepted that lignin mainly contains these structural
units.5 The oxidation of lignin in common solvents was
conducted firstly for the comparison. The results show that the total
yields of phenolic monomers are far from desired value when water,
octane and n -propanol are used as the solvent correspondingly
(Table 2, entries 1-3), where only 22.5 mg g-1 of
phenolic monomers is produced in n -propanol, which can be
ascribed to the limited solubility of lignin in n -propanol (Table
1, entry 3). Besides, the improved yields of phenolic monomers in the
binary systems imply that the addition of octane or water ton -propanol can promote the oxidation of lignin (Table 2, entries
4 and 5). However, there is no obvious increase of phenolic monomers in
the given ternary systems located in multiphase region (Table 2, entries
6 and 7), the limited solubility of lignin in these multiphase emulsions
can be responsible for this situation. But interestingly, the yield of
phenolic monomers in the microemulsion reaches to 90.2 mg
g-1 (Table 2,
entry 8, point c in Figure 3), which gives about 40 to 500 wt. %
increment to those of above
solvents,
indicating that the formation of microemulsion can significantly
intensify this lignin oxidation process. Additionally, the oxidation of
lignin in the above microemulsion in the absence of catalyst was also
studied (Table 2, entry 9). It gives 46.2 mg g-1 of
phenolic monomers, but no products from H, G and S units are detected,
which conforms both the thermal and the above solubilization effects
during lignin depolymerization.
As mentioned, the highest yield of phenolic monomers is obtained from
the above microemulsion (Table 2, entry 8, point c in Figure 3).
Therefore, the mass ratio of n -propanol to octane in
microemulsion was fixed for investigating the influence of
WW (points a-f in Figure 3) during lignin
depolymerization (Table 2, entries 8 and 10-13). It can be found that
the yield of phenolic monomers increases significantly at first and then
has a slight fluctuation with the increase of WW in the
microemulsion region. For example, it is 32.6 mg
g-1 in the absence
of water, while it reaches to the maximum of 90.2 mg
g-1 at the WW of 15.7 wt. %
(Table 2, entry 8, point c in Figure 3). Incidentally, the yields of
products from the transformation of H, G and S units reach to their peak
values at the WW of 15.7, 24.4 and 31.2 wt. %,
respectively (Table 2, entries 8, 12 and 13, points c-e in Figure 3).
Whereas, there is an obvious decrease in phenolic monomer products when
the WW further increases to 43.8 wt. % (Table 2,
entry 14, point f in Figure 3). This decline can be attributed to the
great decrease of interfacial area and the increase of mass transfer
resistance in this multiphase emulsion compared with those in
microemulsions. Similarly, the influence of WP in the
microemulsion with the fixed RW/O of 1.71 (Line Ⅱ in
Figure 3) on the lignin oxidation process was also investigated
(Table 2, entries 8 and 15-17). The yield of phenolic monomers is only
59.7 mg g-1 at the WP value of 53.8wt. % (Table 2, entry 15, point g in Figure 3), which is
partially because this ternary system locates in the multiphase region.
Interestingly, the yield of phenolic monomers reaches to the peak value
when the WP value increases to 75.1 wt. %, the
same point as mentioned above. But it drops to 67.8 mg
g-1 when the WP value further
increases to 86.5 wt .% (Table 2, entry 18, point i in Figure 3).
This can also be ascribed to the change of phase behavior and lignin
solubility in the investigated system (Table 1, entry 6). It should be
noticed that the constructed B.C and W/O microemulsions have similar
efficiency (Table 2, entries 16 and 18, points h and j in Figure 3) for
lignin oxidation compared with those of traditional solvent systems,
while most of the given O/W microemulsions can promote the oxidation of
lignin rationally. In belief, the type and component ratio of
microemulsion are considered as the key factors for this oxidation
process. But, it is interesting to point that the yields of products
from the H unit are at least twice or triple to those from the G or S
unit, although the ratio of H unit in lignin is the lowest, while the
content of H, G and S units in bagasse lignin are 15, 45 and 40%
respectively,41 demonstrating that these systems are
prefer to oxidize H unit rather than other units.
Table 2. Effect of different solvents on the oxidation of
lignin.